1. Field of the Invention
The present invention relates to a mold device for injection molding of synthetic resin having a cavity surface which is alternately heated and cooled.
2. Description of the Related Art
In injection molding of thermoplastics, when the melted resin is filled into a mold cavity having the surface thereof kept at a high temperature, enhancement of quality of the injection molding can be expected in that, for example, the transferring characteristics from the cavity surface to the molded material surface is good, and any noticeable weld line does not appear. Therefore, a heat-cycle method in which the cavity surface of the mold is heated during the filling of the melted resin into the cavity is put into practical use.
As a method for heating or cooling the cavity surface for a short period of time, for example, a method for heating by heated air is disclosed in Japanese Examined Patent Publication No. 22020/1970; a combination of a heating method by an electric heater and a cooling method by cooled water is disclosed in Japanese Patent Laid-open Publication No. 22759/1976; a method for heating by high frequency induction is disclosed in Japanese Patent Laid-open Publication No. 109639/1980; a method for heating by supplying steam into a cavity is disclosed in Japanese Patent Laid-open Publication No. 165229/1982; a method for heating by interposing a heated plate between a cavity and a core is disclosed in Japanese Patent Laid-open Publication No. 79614/1986; a method for heating by halogen bulb is disclosed in Japanese Patent Laid-open Publication No. 42217/1989; and a method for heating a mold surface by an electrically conducting layer is disclosed in Japanese Patent Laid-open Publication No. 265720/1992.
The present inventors have developed a heat-cycle molding method for heating and cooling a cavity surface for a short period of time by a combination of use of steam and use of water and an apparatus for implementing this method as disclosed in, for example, Japanese Patent Application No. 375069/1999. In this heat-cycle molding method, the cavity surface is heated and cooled. Since the heating and cooling are frequently repeated, a careful measure against the thermal strain is provided for the mold construction.
Namely, the heating and cooling channels for heating and cooling the surface of a cavity block is designed in a manner such that a distance between every neighboring channels and a distance between the channels and the cavity surface are designed as appropriate so as to prevent fatigue of the respective members. As the surface of a cavity carved in the cavity block is heated and cooled, space between the cavity block and the mold base is appropriately designed in consideration of a thermal expansion of the cavity block. Further, a corner portion of a hole of the mold base in which the cavity block is housed is rounded at an appropriate curvature.
In such a mold construction, a suitable heat insulator is interposed between the cavity block and the mold base in order to prevent occurrence of a problem such as hampering the rise of temperature of the cavity block due to heat transmission from the cavity block to the mold base. The heat insulator provides some advantages in addition to the above-described effect. That is to say, the heat insulator inserted on the side face of the cavity block is useful for mitigating any thermal strain occurring between the cavity block and the mold base. Further, the heat insulator inserted on the bottom face of the cavity block is effective for reducing contact pressure occurring between respective thermally expanding cavity blocks in the cavity and the core.
In order to produce a molded product having a bored portion like a window, there has been proposed a mold having a bottoming structure therein. Namely, as shown in FIGS. 1 and 2, a bottoming portion 5a is provided on a parting surface of a cavity side mold or a core side mold defining a cavity 1.
Further, in order to produce a molded product having a window, a lattice, or an inverted taper portion at its side portion, there has been proposed a mold having a slide structure. This slide structure consists of a slide core and a slide cavity block provided inside the slide core. The slide cavity block is provided with heating and cooling channels similar to the mold base. For the purpose of avoiding problems caused by either thermal expansion or thermal shrinkage of the mold base and cavity block, there is usually provided a clearance for permitting of thermal expansion.
Nevertheless, in the described structures of the mold according to the prior art, unexpected problems occurred as described hereinbelow.
The following first problem occurs in the above-described first mold structure, i.e., in the bottoming structure of the mold for producing the molded product having the bored portion like a window. When at least one of the cavity surfaces 2 and 3 of the cavity side mold and core side mold, in which the bottoming portion 5a is provided on a parting surface, is subjected to repeated heating and cooling, a crack 9 appears in the mold at a portion located immediately outside the bottoming portion 5a for forming the window, and accordingly a heating or cooling medium oozes out of the heating and cooling channel 4 adjacent to the cavity surface via the crack 9 toward the exterior of the mold.
The following second problem occurs in the above-described second mold structure according to the prior art, i.e., in the slide structure of the mold for producing a molded product having a window, a lattice, an inverted taper portion, or the like in the side portion. Even if a clearance for the thermal expansion or shrinkage of the mold base and the cavity block of the side structure is provided between the slide core and the mold base to avoid the problem relating to the thermal expansion and shrinkage, there is still a problem of galling caused by the thermal expansion.
Therefore, a first object of the present invention is to prevent occurrence of any crack at a portion located immediately outside the bottoming portion of the mold, in a mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled and a bottoming portion enabling production of a product having a bored portion like a window.
A second object of the present invention is to prevent occurrence of any galling between the slide core and the mold base due to thermal expansion, in a mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled and a slide structure.
One feature of the present invention resides in that, in a mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled, a bottoming portion for producing a product having a bored portion is provided on a parting surface of the mold, and a vacant space is also provided in the bottoming portion for reducing contact area. Thus, the contact area of the bottoming portion is small so that it is possible to reduce a contact pressure of the bottoming portion and thermal strain due to a temperature difference between the cavity side mold and the core side mold, and to prevent occurrence of any crack in the mold. The above-mentioned vacant space should preferably be connected to a gas vent.
Another feature of the present invention resides in that, in a mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled, the mold base, a slide core provided in the mold base, and a slide cavity block in the slide core are separately thermally controlled.
A further feature of the present invention resides in that, in a mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled, clearances are provided between a slide core, which is provided in the mold base and has a slide cavity block, and guide rails arranged on opposite side portions of the slide core, and a center rail is provided in the central position of the slide core for guiding the slide core.
In accordance with the above-described constitution of the mold device for injection molding of synthetic resin having a cavity surface alternately heated and cooled, it is possible to prevent galling between the mold base and the slide core of the slide structure.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.